This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A125873 #5 Oct 19 2017 03:15:05
%S A125873 23,67,89,199,331,353,397,463,617,661,727,881,991,1321,1409,1453,1783,
%T A125873 2113,2179,2311,2377,2663,2971,3169,3301,3389,3631,3697,3851,4159,
%U A125873 4357,4621,4951,5281,5347,5501,6337,6469,7129,7393,7547,8317,8713
%N A125873 Prime numbers n such that cos(2pi/n) is an algebraic number of an 11-smooth degree, but not 7-smooth.
%t A125873 Do[If[Take[FactorInteger[EulerPhi[2n+1]][[ -1]],1]=={11} && PrimeQ[2n+1],Print[2n+1]],{n,1,10000}] (*Artur Jasinski*)
%Y A125873 Cf. A125866-A125878.
%K A125873 nonn
%O A125873 1,1
%A A125873 _Artur Jasinski_, Dec 13 2006
%E A125873 Edited by _Don Reble_, Apr 24 2007